Yes, it is true that there are some ticket categories that cannot be altered. However, as has been noted on these boards, sometimes guests encounter CMs who are mistaken about the process of adding days or cannot calculate the correct price difference. In that case, I have found that it is best to simply go to another guest relations office, advice which I have read many times on these boards. For me, the Epcot guest relations office has been a reliable location with knowledgeable CMs, but others’ experiences might differ.